Methylmercury is the most toxic form of mercury and has been responsible for numerous cases of human deaths and toxicity. The recent observation that methylmercury exists in elevated levels in fish from lakes that have been acidified indicates a significant public health threat exists to populations consuming fish from acidified lakes. Methylmercury is produced from inorganic mercury by microorganisms in aquatic sediments, although the in situ mechanism of methylation is unknown. The long-term objectives are to elucidate the in situ mechanism of mercury methylation in freshwater sediments. The specific aims are to use inhibition and stimulation experiments to indicate if methylcobalamin, S-adenosyl methionine, or an incorrect synthesis of methionine is responsible for mercury methylation in freshwater sediments. The importance of these mechanisms in sulfate-reducing bacteria capable of methylating mercury will be examined. Sediments used in this study will be collected from Lake Clara, an oligotrophic acid-susceptible lake, that is known to contain fish with mercury concentrations greater than the FDA action level of 1.0 ug/g. The sulfate-reducing bacteria used in this study will be isolated from Lake Clara sediments and characterized. Compounds known to inhibit or stimulate these methylation mechanisms will be added to sediments or cultures of sulfate-reducing bacteria and their effect on mercury methylation observed. The effect of each addition on overall sediment metabolism (as evidenced by methane production) will also be monitored to determine if the effect on methylation was due to an indirect effect on microbial activity. Methylation will be measured by adding radioactive mercury (II) to sediments and bacterial cultures. The radioactive methylmercury formed during a 1-day incubation will be extracted and quantified by scintillation counting. The accomplishment of the proposed research will provide the first evidence indicating what the in situ mechanism of mercury methylation in aquatic sediments may be. This preliminary evidence will provide a foundation for further research on the mechanism indicated. This will further our understanding about how methylmercury, a toxic compound that poses a significant human health threat, is produced in the environment.